The invention relates generally to methods and materials for disposal of radioactive iodine wastes from nuclear reactor fuel cycles, as well as capture and immobilization of non-radioactive iodine species.
Radioactive 129I is one of the longer-lived fission products (1.6×107 years) resulting from the generation of nuclear energy, and it is also one that is associated with considerable public concern by virtue of the obvious mechanism whereby it may become concentrated in the human body. Historically, 129I was simply discharged to the atmosphere. Currently, iodine is discharged to the ocean (principally the seas around Europe) for isotope dilution with the natural iodine in seawater.
With the growth of research on advanced fuel cycles in the United States and abroad, there is a strong interest in the separations and waste form development for all radioisotopes that are isolated in the developing nuclear cycles. This includes the initial trapping of gaseous iodine radioisotopes, and their incorporation into waste forms. Whether wastes are slated for above ground storage, or underground burial, a serious need is that the radionuclides (129I, in our case) exist in chemical forms that will not be readily dissolved should water gain access to the site.
A second major consideration is that the wastes not exist as powders, since an accident during storage or handling could produce a cloud of radioactive dust with the potential for causing widespread contamination.
A number of research groups have investigated the complex crystal structures of layered bismuth oxy-iodide compounds. In particular, the researchers focused on the subtle, yet related, differences in the topography of the bismuth oxide layers, and the stacking around the iodine complexes located between the layers (see FIG. 1). Due to the high measured stability limits of bismuth carbonates and iodides with respect to saline groundwater, recent research in Canada has focused on the use of individual bismuth oxyiodide compounds as candidates for radioactive iodine waste forms.
Hence, a need exists for improved methods of synthesizing mixed-layer bismuth oxyiodine and oxy-iodate materials for use in the in-situ recovery of radioactive iodine from caustic waste streams and/or final waste form. In particular, we are focused on the use of these mixed-layer Bi—O—I waste forms if repository conditions are at temperatures at, or below, those under which the iodine was initially captured.